Apparatus for brazing and heat treating honeycomb sandwich structures



July 31, 1962 D. c. ROWE 3,047,710

APPARATUS FOR BRAZING AND HEAT TREATING HONEYCOMB SANDWICH STRUCTURESOriginal Filed Oct. 24, 1958 FIG. 1 15 11 lll 1 I I I I I I I I I I I! Il I I I I I I I I I I I I I II I I I I I I! I T] I 7! V? Z7 23 33 15 as37 llllllllllllllllllllllllllllllllllllllllllllllllllllllllllii 31awazsafiazwmmzarzr qly7llllllmlll Illa/111111111 [Il /[Ill drill/AW I$117T/IIIIIIII/IIIII/IIIIIITIIIIII, Illllllllllr DONALD C. ROWEINVENTOR.

United States Patent 3,047,710 APPARATUS FOR BRAZING AND HEAT TREAT- INGHONEYCOMB SANDWICH STRUCTURES Donald C. Rowe, Glen Mills, Pa., assignor,by mesne assignments, to Boeing Airplane Company, Seattle, Wash., acorporation of Delaware Original application Oct. 24, 1958, Ser. No.769,449, now Patent No. 3,011,926, dated Dec. 5, 1961. Divided and thisapplication Sept. 29, 1960, Ser. No. 59,276 3 Claims. (Cl. 21985) Thisapplication is a division of an application filed by me on October 24,1958, Serial No. 769,449 entitled Method and Apparatus for Brazing andHeat Treating Honeycomb Sandwich Structures and now Patent No. 3,011,926, issued December 5, 1961.

This invention relates to improvements in brazing and heat treating, andmore particularly concerns apparatus for brazing and heat treatinghoneycomb sandwich structures.

High speed aircraft require structural material capable of withstandingelevated skin temperatures. To with stand these temperatures, use hasbeen made of stainless steel honeycomb sandwich structures.

However, the economical manufacturing of such honeycomb structures haspresented many problems. Many of the current production methods arecostly and difiicult to control. According to one heretofore proposedmethod, the elements of the honeycomb sandwich structure are placed on agraphite block, and various cover sheets are placed thereon in anindividually formed retort, with the top sheet exerting pressure on theelements by the use of a partial vacuum. The retort is then sealed bywelding the top sheet in place, after which the retort is purged ofimpurities by flooding it with argon. Then the retort is loaded intofurnace which is also flooded with argon to compensate for possibleleaks in the retort welding.

After the retort and its contents have been subjected to the brazing andheat treating cycle, it is removed from the furnace and inserted in arefrigerated box for deep freezing, after which the brazed sandwichstructure is aged, either with or without its supporting retort.

The capital expense is large, primarily because of the cost of furnacesof such large capacity so as to be able to contain and heat both thebrazed sandwich and its enclosing fixture and retort in a controlledatmosphere. Further, additional furnaces are required for the postageingtreatment.

Fixturing costs for the furnace brazed method are expensive in the costof the graphite material, in the assembly of many graphite blocks into asingle fixture in a manner that does not allow distortion with heat, inthe machining of the bolted graphite asesmbly, in the manufacture of theretorts, in the welding of each sandwich structure into an individualretort, and in the relatively frequent replacement required of theretorts and the graphite fixtures.

Processing costs for the furnace braze method are also expensive. Theinert gas (argon) is not reclaimable, and close control of moisture andoxygen content is required. The costs of monitoring the purity of thegas is an expense in addition to the production costs. Processing isalso expensive in terms of the energy required to first heat, then cool,and then heat again the large mass of the combined retort and graphitefixture.

Control difliculties are present in the furnace braze method due to: (1)the distorting of the fixture and retort under heat and pressure; (2)the necessity of maintaining the purity and moisture content of theargon; and (3) the necessity of achieving and maintaining uniformtemperature in the furnace, and through the retort and fixture to thesandwich structure.

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Accordingly, it is an object of this invention to overcome the problemshereirrbefore mentioned.

It is another object of this invention to provide apparatus for brazingand heat treating honeycomb sandwich structure, which invention includesself-contained heating means.

It is another object of this invention to eliminate impurities. in theatmosphere surrounding the brazing and heat treating operation.

It is another object of this invention to provide means whereby thehoneycomb sandwich structures may be made on a mass production basis.

It is another object of this invention to provide means for rapidlycooling the sandwich structure from brazing temperature to a sub-zerotemperature, and for perform ing the aging portion of the heat treatingcycle without removing the sandwich structure from the apparatus of theinvention.

It is another object to produce a sandwich structure having the bestproperties of the materials being brazed.

Other objects and advantages of this invention, including its simplicityand economy, will further become apparent hereinafter and in thedrawings, in which:

FIG. 1 is a view in front elevation and partly in cross section ofapparatus constructed in accordance with this invention;

FIG. 2 is a view in side elevation and partly in section of theapparatus of FIG. 1; and

FIG. 3 is an exploded view of the elements of the apparatus of FIG. 1,and is drawn on an enlarged scale in order to bring out importantdetails.

Although specific terms are used in the following description forclarity, these terms are intended to refer only to the structure shownin the drawings and are not intended to define or limit the scope of theinvention.

Turning now to the specific embodiment of the invention selected forillustration in the drawings, there is shown apparatus for brazing andheat treating honeycomb sandwich structure, which apparatus includes anenclosure 11 and a brazing fixture 12.

Enclosure 11 is adapted to contain a gas such as air and is scalable toform a vacuum chamber. A conduit 13 is provided for evacuating the gasfrom the vacuum chamber of enclosure 11.

Brazing fixture 12 includes a bottom fixture plate 14, a top fixtureplate 15, a top caul or cooling plate 16 positioned below top fixtureplate 15 and above the honeycomb sandwich structure 17, a bottom caul orcooling plate '18 positioned between sandwich structure 17 and bottomfixture plate 14, upper heating strips 21, and lower heating strips 22.

Top fixture plate 15 is provided with a conduit 23 through which acooling liquid or refrigerant may flow, and similarly, bottom fixtureplate 14 is provided with a conduit 24. Bottom fixture plate 14 ismounted on wheels 25 which provide for easily moving the brazing fixture12 into and out of enclosure 11.

Heating strips 21 and 22 are mounted on shafts 26, and the strips 21, 22are kept taut through the action of arms 27 which are mounted on shafts26, and springs 28 which are connected to arms 27 and serve to pullheating strips 21 and 22 taut. Electrical energy is supplied to heatingstrips 21, 22 from the bus bars 31 which are connected to the strips 21,22 by the electrical cables 3-2. Bus bars 31 are connected to a sourceof electrical power (not shown). Honeycomb sandwich structure 17includes a pair of faces 33, 34 with a honeycomb core 35 positionedtherebetween with braze material 36 located between honeycomb core 35and the faces 33, 34. Faces 33, 34 are preferably of stainless steel,honeycomb core 35 is preferably .0015 inch foil, one-quarter inch squarecell spot welded stainless steel, and braze material 36 is preferably92.5 percent silver, 7.3 percent copper, .2 percent lithium (the Handyand Harmon silver braze). Braze material 36 need not be a sheet ofmaterial, but may instead be sprayed on the brazing areas if desired.

Referring to FIG. 3, also positioned between fixture plates 14 and 15are a stainless steel pressure bag 37, thermal insulation blankets 41,42, and electrical-insulation separators 43, 44. Heating elements 21, 22are preferably made of Nichrome, and separators 43, 44 are preferablymade of fibrous silica. Caul plates 16 and 18 have formed therein thepassages 45 through which coolant may flow to provide for integralcooling. For clarity purposes, the structure holding the fixture plates14 and 15 in fixed relation to each other has not been shown. Any typeof holding structure may be employed. Examples of such are shown on page158 of The American Machinist of March 25, 1957, and Figure 26 of TheAmerican Society for Metals (Los Angeles Chapter) publication of LectureNo. 3 (November 7, 1955).

In the method of the present invention, a stainless steel honeycombsandwich structure 17 is assembled together by taking the thin stainlesssteel faces 33, 34, the honeycomb core 35, and the braze material 36 andtack brazing the elements together. Then sandwich structure 17 is placedon top of bottom fixture plate 14 with its caul plate 18 and heatingstrips 22. Next, the top fixture plate 15, with its caul plate 16,heating strips 21, and pressure bag 37 is placed on top of honeycombsandwich structure 17. Then sandwich 17 and brazing fixture 12 areplaced in the vacuum chamber formed by enclosure 11 and the gas withinthe chamber is evacuated. The pressure within bag 37 is initially abouttwenty-nine inches of mercury. This is too much pressure for elevatedtemperatures, so at this point in the process, the bag 37 is partiallyevacuated to bring the pressure down by approximately twenty-seveninches of mercury, or to one and one-half p.s.i. absolute. Next, theheating strips 21 and 22 are energized to apply heat to faces 33, 34 ina uniform manner. Sandwich 17 is heated to brazing temperature, usuallyabout 1710" F., and held there for approximately ten minutes. Then thesandwich is rapidly cooled by dc-energizing heating strips 21, 22 andsending a refrigerant through the passages 45 of the caul plates 16, 18,and, if desired, also through the conduits 23, 24 of the brazing fixture12. This brings the temperature of sandwich 17 to ambient within tenminutes, after which the sandwich 17 is cooled to sub-zero temperaturewithin thirty minutes. Sandwich 17 is held at about minus 100 F. forapproximately eight hours. After this, sandwich -17 is rapidly heated toa heat treating temperature of about 950 F. and held there forapproximately ninety minutes to complete the heat treating cycle. Thenthe sandwich 17 is cooled to room temperature at a uniformly decreasingrate.

Insulation blanket 41 provides thermal insulation be tween heatingelement 21 and top fixture plate 15, while insulation blanket 42provides thermal insulation between heating strips 22 and bottom fixtureplate 14. Strips 21, 22 are electrically insulated from sandwich 17 bythe thin sheets of fibrous silica 43 and 44.

The pressure within the vacuum chamber formed by enclosure 11 is reducedto 100 microns, while the pressure bag 37 is maintained at any desireddifferential pressure with respect to the vacuum chamber up to a fullatmosphere. Caul plates 16, 18 serve to maintain the desired contour ofthe sandwich 17.

The advantages of the invention are numerous. For example, in brazing ata temperature of 1710 F., the temperature of brazing fixture 12 neverexceeds 400 F. Expansion, distortion, and deterioration of both thebrazing fixture 12 and the sandwich '17 are kept to a minimum. Further,there is no necessity to use or to have a furnace, thereby cuttingappreciably the capital expenditure for the apparatus.

The amount of mass to be heated is reduced to such an extent that astandard one foot by two foot sandwich can be brazed with less than 10kva., whereas previously proposed conventional furnace brazing whichrequires graphite fixtures and a retort would use 200 kva. Accordingly,power costs are reduced.

Since the temperature of pressure bag 37 never exceeds 400 F., it islong lasting and does not require frequent replacement.

Both plates 14, 15 of brazing fixture 12 and the caul plates 16, 18 areWater or refrigerant cooled to reduce the cooling cycle time, and tocomplete the deep-freeze and aging portion of the heat treating cycleWithout removing sandwich 17 from fixture 12 and without removingbrazing fixture 12 from the vacuum chamber formed by enclosure 11. Thissystem thus lends itself to complete programmed automation.

The apparatus of the present invention does not require the use ofargon, therefore eliminating the cost of the argon and its controlproblems.

Maintaining the vacuum within enclosure 11 acts to remove any gasesproduced by the heating of the sandwich 17, or any of the elements ofthe brazing fixture 12, and also removes any residual contaminants.

The apparatus of this invention produces sandwich structures of anydesired shape, such as flat, curved, or tapered, and produces sandwichstructures which exceed the current size requirements. Additionally,because of the small amount of mass to be heated and cooled, the cycletime is considerably reduced. Further, uniform temperature is maintainedon sandwich structure 17 even though it may contain edge members orinserts. This is done by controlling the placement, the current flow,and the resistance of the individual Nichrome heating strips 21, 22.

The apparatus of the present invention is extremely flexible. The smallamount of mass to be heated provides for rapid heating, and thus givesmaximum flexibility to this portion of the brazing, heat treating, andaging cycles. The small amount of mass to be heated also provides forrapid cooling through forced cooling introduced into the caul plates 16and 18.

It has been found that honeycomb sandwich structure 17 made inaccordance with this invention has uniform filleting, complete corebonding, and no intergranular penetration. Waviness and indentationshave been .003

or less, and the bow has been less than .020".

It is to be noted that brazing fixture 12 is thermally insulated formaintaining its temperature at not more than 400 F. so that it can bemade of an inexpensive, rigid material.

It is to be understood that the form of the invention herewith shown anddescribed is to be taken as a preferred embodiment. Various changes maybe made in the shape, size, and arrangement of parts. For example,equivalent elements may be substituted for those illustrated anddescribed herein, parts may be reversed, and certain features of theinvention may be utilized independently of the use of other features,all without departing from the spirit or scope of the invention asdefined in the subjoined claims.

Having thus described my invention, I claim:

1. Apparatus for brazing honeycomb sandwich structures, said apparatuscomprising an open brazing fixture having top and bottom fixture platesfor receiving therebetween a honeycomb sandwich structure comprising ametal honeycomb core between two thin metal sheets; and a vacuum chamberfor receiving said brazing fixture and honeycomb sandwich structure,said brazing fixture including fiat-surfaced cooling plates and flatstrip heating elements positioned between said honeycomb sandwichstructure and said top and bottom fixture plates; means for evacuatingsaid chamber to form substantially a vacuum therein; a flexible metalpressure bag positioned immediately beneath said top fixture plate formaintaining a controlled positive pressure on said honeycomb sandwichstructur sufficient to clamp said sandwich structure without crushingthe core thereof; electrical means for generating high temperature heatin said heating elements for brazing said honeycomb structure; and meansfor passing refrigerant coolant through said coolant plates for rapidlycooling said honeycomb sandwich structure.

2. Apparatus for brazing and heat treating stainless steel honeycombsandwich structures comprising: an open brazing fixture having a pair ofspaced-apart fixture plates adapted for receiving said sandwichstructure therebetween; a flexible metal pressure bag between one ofsaid fixture plates and said sandwich structure; electrical heatingelements positioned between each of said fixture plates and saidsandwich structure for applying heat substantially uniformly to bothfaces of said sandwich-structure and adapted for heating said sandwichstructure to a high brazing temperature of the order of 1700' R; heatinsulating means between each of said fixture plates and said heatingelements for maintaining said fixture plates at a maximum temperature ofthe order of 400 F.; coolant plates positioned between each of saidfixture plates and said honeycomb sandwich structure for receivingrefrigerant coolant and adapted for reducing rapidly the temperature ofsaid sandwich structure from said high brazing temperature; a vacuumchamber for receiving said fixture containing said sandwich structure;and means for substantially completely evacuating said chamber to about100 microns while at the same time maintaining a difierent-ial pressurein said pressure bag to maintain the pressure on said sandwich structureat substantially less than 14.7 pounds per square inch.

3. Apparatus for brazing stainless steel structures, said structurecomprising a stainless steel honeycomb core between two thin stainlesssteel sheets; a brazing fixture open at both ends and at both sides andhaving spaced-apart top and bottom fixture plates; cooling plates andheating elements positioned between said honeycomb sandwich structureand said top and bottom fixture plates; a flexible stainless steelpressure bag positioned immediately beneath said top fixture plate; avacuum chamber for receiving said brazing fixture; means for evacuatingsaid chamber to about 100 microns While controlling the pressure in saidpressure bag for maintaining a controlled positive pressure on saidhoneycomb sandwich structure sufficient to clamp said sandwich structurewithout crushing the core thereof; electrical means for generating hightemperature heat of about 1700 F. in said heating elements for brazingsaid honeycomb structure; and means for passing refrigerant 20 coolantthrough said coolant plates for rapidly cooling said honeycomb sandwichstructure.

References Cited in the file of this patent UNITED STATES PATENTS853,351 Fulton May 14, 1907 2,693,636 Simpelaar Nov. 9, 1954 2,984 732,Herbert May 16, 1961

